Recording/playback devices that record digitally broadcast digital contents on a recording medium, for example, a recordable DVD (Digital Versatile Disk), a memory card, etc., have become common. Specific examples of recording/playback devices include DVD recorders, mobile telephones supporting 1 segment broadcasting video recording, etc. Also, services for digitally distributing digital contents such as movies or music to a recording device over a network, and recording the digital contents on a recording medium with use of the recording device have become common. Specific examples of recording devices include KIOSK terminals, personal computers, etc. Furthermore, digital content distribution services for digitally distributing digital contents over a network to playback devices that play back the digital contents have become common. Specific examples of playback devices include music players, mobile phone terminals for displaying movies, etc.
In this case, technology to prevent a digital content from being copied to another recording medium and played back after the digital content has already been recorded on a recording medium once is necessary for protecting rights of copyright holders of digital contents.
According to patent literature 1, a unique medium-specific number that is specific to the recording medium and cannot be rewritten is stored on the recording medium. An apparatus that grants permission generates a medium-specific key according to the medium-specific number of the recording medium, encrypts, with use of this medium-specific key, a decryption key for decrypting the encrypted data, and writes the encrypted decryption key in the recording medium as permission information. A user apparatus reads the medium-specific number from the recording medium and generates a medium-specific key according to the read medium-specific number. Next, the original decryption key is generated by decrypting the encrypted decryption key that is the permission information with use of the medium-specific key. Then, the user apparatus decrypts the encrypted data with use of the decryption key to generate plain text electronic data.
According to this technology, even if a user copies encrypted data and permission data from an authorized recording medium to an unauthorized recording medium and attempts to decrypt the encrypted data from the unauthorized recording medium, the medium-specific number of the authorized recording medium cannot be copied to the unauthorized recording medium. Accordingly, the user cannot acquire the medium-specific number from the unauthorized recording medium, and the encrypted decryption key that is the permission information cannot be correctly decrypted. As a result, the user cannot correctly decrypt the encrypted data.